Optical heterodyne communication systems using angle modulation generally degrade in the presence of frequency or phase noise related to the finite emission linewidths of the transmitter and local oscillator lasers. This is particularly severe for systems which use semiconductor diode lasers. A three frequency heterodyne technique has been proposed by Vincent Chan to cancel this effect: "Differential Frequency Modulation/Demodulation for Coherent Optical Communications," IEEE International Symposium on Information Theory, Brighton, England, June 23-25, 1985. Such a system has been demonstrated by Bondurant et al. in "Frequency Noise Cancellation in Semiconductor Lasers by Non-Linear Heterodyne Detection," Optics Letters, Vol 11, page 791, December 1986.
In the three frequency heterodyne system, a reference signal is transmitted with the modulated data carrying signal. These signals are derived from the same transmitter laser and are frequency shifted relative to each other. In FIG. lA, the reference frequency is illustrated as the frequency f.sub.R of the laser and the carrier of the modulated signal is illustrated as f.sub.R +f.sub.S. No attempt is made in FIG. 1 to illustrate the frequency distribution of the modulated signal. Phase noise of the reference and carrier frequencies is illustrated. In the three frequency heterodyne technique, the third frequency f.sub.LO of a local oscillator is introduced at the receiver. That local oscillator signal is mixed with the incoming signal comprising f.sub.R and f.sub.R +f.sub.S to produce two intermediate signals at frequency f.sub.IF and f.sub.IF +f.sub.S where f.sub.IF equals f.sub.R -f.sub.LO. These two intermediate frequency signals carry identical phase noise originating from the transmitter laser and the local oscillator laser. By then mixing those two signals, a clean carrier signal without the noise from the laser and local oscillator is obtained at the difference frequency f.sub.S, and noisy signals are obtained at twice the intermediate frequencies. The noisy signals can then be filtered out by a low-pass filter, and the data modulation signal at the carrier frequency f.sub.S can be obtained by use of a suitable demodulator.
Bondurant et al. demonstrated that the three frequency heterodyne detection scheme was feasible for optical systems. However, the transmitter, which utilized acousto-optic modulators, was impractical for large-scale applications due to its opto-mechanical alignment complexity and slow speed.